Many types of semiconductor devices are made in much the same way. A silicon wafer is masked, doped, and etched to produce a plurality of die on the wafer. Each die is separated then attached to a lead frame, with several die attached to each lead frame. The lead frame forms the input/output leads which will allow data to be transferred between the die and the electronic device into which the semiconductor components will be installed.
After the die are attached to the lead frame, the die are encapsulated in a protective layer of plastic material. The equipment which molds the plastic around the die comprises several parts. A bottom mold chase receives a bottom cavity bar, the cavity bar comprising a void which will be filled with plastic. A lead frame having die is placed on the bottom cavity bar, and a press holds a top cavity bar, which is received in a top chase, against the bottom cavity bar. The top and bottom cavity bars form a top and bottom void around the die. All this equipment together forms a mold base. Molten plastic is pumped into the top and bottom cavities to form the top and bottom halves of the package around the die. Each of the plastic packages formed on the lead frame are separated and the excess plastic is trimmed away from the lead frame. The plurality of bodies formed on each lead frame are separated, and the metal is trimmed and formed to produce the final semiconductor component.
The mold chases, which are rectangular in shape, have a hole jig ground in each corner. The jig grinding presently costs approximately $50 for each hole, adding $200 to the cost of each chase. Dowels on the mold base are received by the holes in the corners of the chases, the chases being aligned in this manner. After the dowels receive the chase, the chase is bolted to the mold base to hold it firmly in place. The mold base can receive several top and bottom chases at once, thereby increasing the productivity of the transfer molding equipment.
Semiconductor manufacturing facilities produce several package types. When a new type of package is to be produced the chases are removed and a different type of chase is installed on the dowels. Several different package types may be produced weekly.
Each time the chases are removed there is friction between the chases and the dowels. Since the dowels are made of a much softer material than the chases, metal is worn away from the dowels. Due to forces involved with removing and replacing chases, the dowel holes on the chases can wear and shift location or become damaged, thereby contributing to subsequent alignment problems. Over time the dowels become smaller in diameter and the chases are somewhat loose on the dowels, thereby producing incorrect alignment between the top and bottom chases. In addition, allowable tolerances in the placement of the holes in the corners of the chases can force the dowels further apart or closer together, thereby wearing the dowels even faster and causing the base of a dowel to become loose. The loose dowels cause misalignment of the top and bottom chases. This misalignment eventually makes the semiconductors produced with the worn dowels unusable, and the dowels must be replaced at no small cost.
Another drawback with the dowel arrangement is that it makes replacing the top and bottom mold chases time consuming. Each chase must be removed from the dowels on the mold base, and different chases installed which takes a skilled operator approximately 20 minutes of downtime.